It's not possible to credit note a paid bill in DPS. Please follow the steps below which is a workaround for this.
From the DPS home screen go to Client Postings - Bills
Post a bill use the Add Profit Cost button, but enter this figure as a minus
You would now have a credit bill posted, which can either be used to:
Allocate against a newer outstanding bill
Transfer from the office to the client account using payment type (OTC) - which in turn would then allocate against the negative bill.
Reimbursed directly from the office account to the client using payment type (ODR) - which in turn would then allocate against the negative bill.